1. Field of the Invention
This invention relates to a novel class of quaternary pyridinium compounds useful for selectively inhibiting acetylcholinesterase (AChE) and to their use as nerve agent antidotes and prophylactics. We have found that certain quaternary pyridinium derivatives can offer in vivo protection against the nerve agents Soman (GD) and Tabun (GA) and therefore they can also offer protection against other organophophate poisons. We have also found that certain quaternary pyridinium derivatives can effectively increase the rate of survival of experimental animals exposed to the nerve agents GD and GA and therefore they can also serve as antidotes against other organophosphate poisons. All of the active pyridinium compounds are good AChE inhibitors in vitro and therefore they can also be used to treat conditions where a cholinergic deficit leading to pathology can be ameliorated by administration of an anticholinesterase agent. All of the pyridinium compounds described herein can have additional beneficial effects in living organisms by virtue of their resemblance to the natural neurotransmitter acetylcholine and their ability to act on cholinergic receptors.
2. Description of the Related Art
Development of an effective antidote to organophosphorus (OP) nerve agents has been a goal of medicinal chemists since the development of Tabun in Germany in 1937. OP agents react covalently with the active site serine hydroxyl group of acetylcholinesterase (AChE) to form a stable phosphonyl ester which requires up to 30 days for hydrolysis (Main, A. R. In Biology of Cholinergic Function; Goldberg, A. M., Hanin, I., Eds.; Raven Press: New York, 1976; pp. 269-353, incorporated herein by reference). Excess acetylcholine accumulates and causes severe neurological imbalance, respiratory paralysis, and death (Koelle, G. G. In The Pharmacological Basis of Therapeutics; Goodman, L., and Gilman, A., Eds.; MacMillan: New York, 1975; p. 404, incorporated herein by reference). Pyridinium aldoximes (2-PAM and HI-6, see structures below) are currently used in conjunction with an acetylcholine antagonist (atropine) to reactivate the phosphonylated serine hydroxyl group of AChE after poisoning occurs (Leadbeater, L., Inns, R. H., and Rylands, J. M. Fund. App. Toxicol. 1985, 5, S225, incorporated herein by reference). This treatment is ineffective against GD and GA because the initially formed enzyme bound alkyl phosphonate esters undergo rapid aging with loss of the alkoxy side chain to form a negatively charged phosphonyl mono ester (Wolthuis, O. L., Berends, F., and Meeter, E., Fund. Appl. Toxicol. 1981, 1, 183, incorporated herein by reference). The negative charge on this mono ester repels nucleophiles which are required to dephosphonylate the enzyme.
Other approaches to OP antidotes have dealt with changes in the heteroaromatic ring system in order to improve the reactivation of OP-inhibited acetylcholinesterase (Bedford, C. D., Harris, R. N., III, Howd, R. A., Goff, D. A., Koolpe, G. A., Petesch, M., Koplovitz, I., Sultan, W. E., and Musallam, H. A., J. Med. Chem. 1989, 32, 504; Bedford, C. D., Harris, R. N., Howd, R. A., Goff, D. A., Koolpe, G. A., Petesch, M., Miller, A., Nolen, H. W., III, Musallam, H. A., Pick, R. O., Jones, D. E., Koplovitz, I., and Sultan, W. E., J. Med. Chem. 1989, 32, 493; Benschop, H. P., Van der Berg, G. R., Van der Hooidonk, C., DeJong, L. P. A., Kientz, C. E., Berends, F., Kepner, L. A., Meeter, E., and Visser, R. P. L. S., J. Med. Chem. 1979, 22, 1306; Grifantini, M., Martelli, S., and Stein, M. L., J. Med. Chem. 1973, 16, 937, incorporated herein by reference). ##STR2##
Since it is difficult to reactivate aged phosphonylated AChE, prophylactic treatment with AChE inhibitors may be the preferred alternative. AChE is protected from nerve agents by preadministration of an AChE carbamylating agent such as pyridostigmine or physostigmine. These carbamates react covalently with AChE at the active site serine, but the carbamylated enzyme is hydrolyzed quickly to active enzyme (30 min.) while phosphonylated AChE requires several hours or days for regeneration of active AChE (Main, A. R., vide supra). Blocking AChE for a short period of time allows destruction of the OP agent by uncatalyzed hydrolysis or reaction with enzymes such as carboxylesterase or pseudocholinesterase (Harris, L. W., Stitcher, D. L., and Heyl, W. C., Life Sci. 1980, 26, 1885, incorporated herein by reference). Unfortunately, physostigmine is very toxic and small doses can be lethal. Furthermore, physostigmine is unstable and would rapidly decompose in the battlefield. Both carbamates are effective only when combined with several other drugs (Heyl, W. C., Harris, L. W., and Stitcher, D. L., Drug Chem. Toxicol. 1980, 3, 319, incorporated herein by reference).
Other approaches to prophylaxis against OP poisoning have involved the use of cyclic organophosphates to temporarily block AChE (Ashani, Y., Leader, H., Raveh, L., Bruckstein, R. and Spiegelstein, M., J. Med. Chem. 1983, 26, 145, incorporated herein by reference), reduction of AChE levels by inhibition of choline acetyl transferase (Gray, A. P., Platz, R. D., Henderson, T. R., Chang, T. C. P., Takahashi, K. and Dretchen, K. L., J. Med. Chem. 1988, 31, 807, incorporated herein by reference), and the use of antimuscarinic .alpha.-adrenergic agonists (e.g., clonidine) to block the release of acetylcholine from presynaptic nerve terminals (Buccafusco, J. J. and Aronstam, R. S., J. Pharmacol. Exp. Ther. 1986, 239, 43, incorporated herein by reference).
Many oxime, semicarbazone, hydrazone and acyl hydrazone derivatives of 2-formyl-3-hydroxy-1-methylpyridinium chloride described herein incorporate both, a nucleophile to displace the phosphonate from the serine hydroxyl group of AChE and an additional hydrogen bonding group (hydroxyl) to neutralize the negative charge on the aged phosphonate mono ester. In addition, the parent 3-hydroxypyridinium derivatives were converted into various carbamates since these are pyridostigmine analogs and can react covalently with AChE and thus provide protection from OP agents.